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Hypertensive kidney. Gross specimens of a normal healthy kidney (bottom) and a kidney damaged by high blood pressure (hypertension, top). The damaged kidney is smaller and has a bumpy surface due to scar tissue. The kidneys filter waste products from the blood. Sustained high blood pressure damages the individual filtering units (nephrons), which become replaced with fibrous scar tissue, making them unable to function. As more nephrons are damaged toxic substances start to build up in the blood, eventually leading to kidney failure, which can be fatal. Treatment is aimed at reducing blood pressure. In some cases dialysis may also be needed.